Silicon (Si) is essential for bone formation, and participates in several other metabolic processes in mammals; in diatoms it is essential for cellular metabolism including synthesis of cAMP and DNA; as asbestors and in dusts, Si is the causative agent in fibrotic lung diseases and cancer. Despite its importance, however, research on Si biochemistry is in its infancy, due in part to inherent methodological difficulties. Using the diatom as a paradigm has overcome many of the obstacles and yielded very consequential data on the regulatory role of Si in this organism. The proposed research will extend our studies on Si metabolism, regulation of gene expression, and mineralization in vivo: (i) Characterization of silicate ionophores will be completed and their relation to the silicate transport carrier and other intracellular processes will be determined. In addition, studies on their chemical structure and synthesis of compounds with ionophoretic activity will be carried out. This will not only greatly extend our knowledge of Si transport but could also provide a means of creating totally Si-free media, essential for biological Si- research. (ii) Recombinant DNA techniques will be used to isolate and characterize Si-responsive genes as to response pattern, presence and DNA sequence of regulatory regions, and function. In addition, these techniques will be applied to our studies of Si-regulation of DNA replication. (iii) In our continuing electron microscope studies, the morphological forms of silica in structurally complex cell walls will be examined; the occurrence of an organic matrix in the cell wall will be investigated; and distribution of intramembrane particles in the silicalemma during silica deposition will be determined. The presence of an organic matrix and its relation to silica deposition could radically change present concepts of the diatom cell wall, suggest morphological resemblance to bone matrix, and contribute to our knowledge of bone formation and its Si requirements. In view of Si's importance as both a metabolite and a pathogenic agent, any increases in our knowledge of its mode of action at the molecular level is valuable input to health-related research.